Ultraviolet-curable coating compositions are receiving greater attention in industry because they cure rapidly with a minimum of volatiles being expelled as the liquid coating composition converts to a solid solvent-resistant film. Such compositions frequently include one or more polyacrylates in combination with one or more photoinitiators and/or photosensitizers to enable the polyacrylates to respond to the ultraviolet radiation which induces cure. Two common defects of such coatings have been poor adhesion of the cured coatings to the substrate and poor abrasion resistance. Adhesion is especially poor when the substrate is a plastic possessing a basic surface, as is found in polycarbonate resins and ester-plasticized polyvinyl chloride resins. These resins are frequently used to provide molded products.
As a matter of interest, the basic nature of these plastic surfaces is set forth by F. W. Fowkes in "Acid-base Interactions in Polymer Adhesion", see the text "Physicochemical Aspects of Polymeric Surfaces" Vol. 2, Edited by K. L. Mittal, Plenum Press, N. Y. pages 583-603. The plastic surfaces having a basic nature are normally provided by the presence in the plastic of unpaired pairs of electrons as is provided by ester, ether, amino and oxirane groups. The resins of particular interest in this invention are those containing the carbonyl group, as is provided by the ester group.
It is desired to coat these basic-surfaced plastic moldings to improve their solvent resistance, and to do so with ultraviolet-curable coatings, but this has not been simple because of the poor adhesion and poor abrasion resistance which characterize such coatings.